Recently, the rapid development of smaller, lighter, and higher performance electronic and communication equipment has required the development of high performance and large capacity batteries to power such equipment. The demands for large capacity batteries have led to investigations into rechargeable lithium batteries. Positive active materials for rechargeable lithium batteries generally use lithium-transition metal oxides, and negative active materials generally use crystalline or amorphous carbonaceous materials or carbon composites. The active materials are coated on a current collector with a predetermined thickness and length, or they are formed as a film to produce electrodes. The electrodes together with a separator are wound to produce an electrode element, and the electrode element is inserted into a battery case such as a can followed by insertion of an electrolyte to fabricate a battery.
The electrolyte includes lithium salts and organic solvents. The organic solvents may be mixed solvents of between two and five components of cyclic carbonates such as ethylene carbonate or propylene carbonate, or linear carbonates such as dimethyl carbonate, ethylmethyl carbonate, or diethyl carbonate. However, these solvents are known to severely expand at high temperatures, causing a swelling phenomenon. The swelling phenomenon is partly manifested in a battery by gas generated due to decomposition of the electrolyte at high temperatures in the battery.
Such a swelling phenomenon can be reduced in lithium polymer batteries compared to lithium ion batteries. However, the use of a polymer electrolyte with a nickel-based positive active material (e.g. LiNiMO2, where M is selected from Co, Mn, Al, P, Fe or Mg) has generally been ineffective.
Several attempts to use solvents with a high boiling point and a high dielectric constant, such as γ-butyrolactone, have been promising. Conventionally, the high dielectric constant solvent is generally used together with ethylene carbonate, which results in an extremely high viscosity with poor wettability of the separator. In another attempt, solvents with low boiling points and low dielectric constants have been used. However, these attempts still have problems associated with high swelling (Japanese Patent Laid-Open No. 2000-235868, U.S. Pat. Nos. 5,079,109, 5,272,022, 5,552,243, 5,521,027, 6,117,596, and 5,851,693, and “New thin lithium-ion batteries using a liquid electrolyte with thermal stability” Journal of power sources, 97-98, 677-680(2001), Notio Takami et al.)
Other attempts to inhibit the swelling phenomenon are in U.S. Pat. No. 4,830,939 disclosing a liquid electrolyte containing a polyethylenically unsaturated monomeric material or a prepolymeric material, and U.S. Pat. No. 4,866,716 disclosing a cross-linked polyether which is a product of a vinyl-ether. In addition, U.S. Pat. No. 4,970,012 discloses that a polymeric solid electrolyte includes crosslinked molecules of a radiation-cured substance of a cinnamate ester and polyethene oxide, and U.S. Pat. No. 4,908,283 discloses that a polymeric electrolyte includes a cured product of an acryloyl-denaturated polyalkylene oxide.
Such a swelling phenomenon is especially severe in batteries with a mixture of a lithium cobalt-based compound and a lithium nickel-based compound which exhibits higher capacity than other compounds.